This invention relates to a method of manufacturing tablets consisting of two or more separate layers of material, and more particularly relates to a method of independently monitoring the amount (weight) of powder comprising each layer and making controlled adjustments to the filling mechanisms which determine those weights in order to maintain each layer within proper weight limits.
Monitoring and controlling the weights of single layer tablets produced on rotary presses is a well established art. Typically, tablet weights are determined by measuring the amount of force necessary to compress or compact a powder or granulation into a solid tablet. For a given product, the compaction force necessary to obtain the desired tablet thickness and hardness when the press is making tablets having the correct weight is known. Overweight tablets resulting from excessive material in the die cavities will cause higher than normal compression forces. Low weight tablets resulting from insufficient powder being present will cause below normal forces. Currently, a whole array of electronic and mechanical hardware and computer software is available from various vendors for monitoring tablet compression and making adjustments to press powder fill mechanisms as required to keep tablet weights within specified limits. For example, over and underweight tablets are typically detected in tablet presses through use of a strain gauge transducer (or related mechanism) for measuring the forces in the opposing punches. Such a strain gauge transducer is disclosed in U.S. Pat. No. 3,791,205 issued on Feb. 12, 1974. U.S. Pat. No. 3,734,663, issued May 22, 1973, discloses a control circuitry that monitors, and if necessary, changes the amount of powder placed in the die in response to measured tablet compressive forces, as detected by changes in resistance in a strain gauge.
Rotary tablet presses having two complete sets of the cams, feeders, pressure rolls, etc. necessary to form and discharge tablets are referred to as double sided machines. Since two tablets per station of tooling in the turret are produced for each revolution of the turret, these machines have twice the output of a single sided machine operating at the same turret speed. As such, double sided presses are normally used for products manufactured in large volumes. These machines, however, are also used to make tablets having two separate layers of material.
In two layer tablet production, the die cavities in the turret are filled to the desired level with the first layer material as they pass underneath the feeder on one side of the machine. This first layer material is then compressed, usually at a greatly reduced force level than what is normally used in single layer tablet making. After compression, the dies pass under the feeder on the other side of the machine and are filled to the appropriate level with second layer material. The dies, with both first and second material in them, then move to the final compression rolls where solid tablets are formed. Tablet ejection from the die cavities and scrape off follow.
Monitoring and controlling the weight of first layer material in a two layer operation is almost identical to single layer production. In general, the only difference is that a much lower compression force is used to tamp the material prior to introducing the second layer material. This first layer tamp is intended to make room in the die cavity for second layer material and provide a sharp demarcation line between layers.
Second layer weight monitoring and control is much more problematic since the compression signal generated during final tablet compression is affected by both first and second layer weight variations. Lower than target force signals may be the result of low first and/or low second layer weights. They could even result from very low first layer weights and high second layer weights or vice versa. Compounding this situation is the fact that the compression properties of each layer may be very different.
Currently two approaches are used to control tablet weights for a two layer process. The first is to monitor and control only the first layer side. Second layer (final) compression forces may or may not be monitored for the purpose of rejecting bad tablets but no automatic adjustments are made to the second layer fill mechanism. The second approach involves actual control of both first and second layer material. First layer weights are monitored and controlled the same way as for single layer production. Second layer control is achieved in much the same way except that the second layer control loop must be desensitized to the measured compression force errors (deviations from set point) arising from first layer weight variations. This is accomplished by decreasing the second layer control loop gain.
Although the second control scheme described above can result in improved second layer weight control, its success is highly dependent upon the degree to which the first layer is being controlled. Even for well behaved first layer materials, weight control of the second layer is less than optimum because the control loop gain must be reduced so as not to overact to the variation, or noise, from the first layer.
Accordingly, it is an object of this invention to provide a tablet press control mechanism suitable for the manufacture of a multilayer tablet whereby tablet compression information for each layer is precisely monitored and a method therefore.
It is a further object of this invention to provide a method for the manufacture of a multilayer tablet that permits the monitoring and controlling of the compression information for each individual layer independently.
It is a further object of this invention to provide a method for the manufacture of a multilayer tablet that permits a quality determination (i.e., a rejection decision) of the tablet based on the compression information for each layer of the tablet independently. That is, a rejection decision may be made based on the compression information for any layer independent from the compression information for the remaining layers.
It is an additional object of this invention to provide a method for the manufacture of a two-layer tablet utilizing a double-sided rotary multilayer tablet press that precisely monitors tablet compression information for each layer independently.